Gas chromatography

ABSTRACT

&#39;&#39;&#39;&#39;Tailing&#39;&#39;&#39;&#39; of peaks in gas chromatography is reduced by providing a bleed of gas through places in chromatographic apparatus which part of a sample may enter and thus be held up.

United States Patent Deans Dec. 18, 1973 GAS CHROMATOGRAPHY [75]Inventor: David Robert Deans, References Cited stockton-on-Tees, EnglandUNITED STATES PATENTS [73] Assignee: imperial Chemical Industries3,656,277 4/1972 Slingerland 55/67 London England Buchtel [22] Ffled:1971 Primary Examiner-Charles N. Hart [21] Appl. No.: 193,708Attorney-Cushman, Darby & Cushman [30] Foreign Application Priority Data[5 7] ABSTRACT NOV. 9, 1970 Great Britain 53,] i7/7O Tailing of peaks ingas hromatography is reduced by providing a bleed of gas through placesin chro- U-S- Cl. matographic apparatus part of a amplg may int. enterand thus be up [58] Field of Search 55/67, 197, 386;

73/23.l, 422 GC 5 Claims, 1 Drawing Figure GAS CHROMATOGRAPHY THISINVENTION relates to gas chromatography. In gas chromatography a carriergas is passed through a gas chromatographic column, a sample containinga number of components is injected into the carrier gas and thecomponents separate as they pass through the column and are detected inthe effluent from the column as peaks in a detector signal.

It is commonly observed that the peaks corresponding to componentsemerging from the column have tails", i.e., the peaks are notsymmetrical, a small' quantity of the component emerging over a periodafter the passage of the main part of the peak. Such tailing complicatesinterpretation of the results, especially when a peak of the secondcomponent occurs during passage of a tail.

Tailing is in some cases due to the nature of the packing in thechromatographic column; in particular packings of which the partitionco-efficient of the component between the packing and the vapour phasedeclines with increasing concentration of the component in the vapourphase (i.e., a smaller proportion of the component is sorbed as itsconcentration increases) show this effect. However, we haye found thattailing" is often caused also by geometrical factors.

In general we find that if there is any unswept volume at any point towhich sample is carried, sample which enters such volume diffuses fromit over a period and thus causes tailing."'By unswept volume is meant avolume within a gas chromatographic apparatus communicating with the gaschromatographic stream but through which no major flows of the stream orof carrier gas occur during normal operation of the apparatus. Suchunswept volume includes not only recesses but also volume caused byangles especially acute or right angles in conduits along which the gasis passing.

It is unfortunately a matter of extreme difficulty to produce a gaschromatographic system which does not contain unswept volume, and thisproblem is aggravated by the normal method of sample injection which isused. In this, a liquid sample is injected through a septum using ahypodermic syringe and expelled as droplets into the carrier gas stream.This involves the presence of unswept volume in the neighbourhood of theseptum, and since it is in general necessary to apply heat to vaporisethe sample and septa are normally not very heat-resistant, it is usualto position the septum at a considerable distance from the point ofinjection and to bring the sample to the required position by pushing aconsiderable length of the needle of the hypodermic syringe through theseptum until the tip of the needle is in the required position. Thisdesign involves the presence of unswept volume. The problem isparticularly important at this point because expansion of the sample onvaporisation tends to drive sample into unswept volumes even if they areup-stream from the point of injection. Unswept volumes tend to occuralso where pieces of tube are united to one another or to otherapparatus by means of a conventional tube coupling.

This invention is based on providing small bleeds of gas through unsweptvolumes from or to the chromatographic gas stream, so that any sample orcomponent entering the unswept volumes will bleed away, preferably tothe atmosphere, rather than contributing to a tail, or so that flow ofgas to the chromatographic gas stream will prevent sample entering theunswept volume or carry it quickly back to the said stream.

The invention enables relatively large unswept volumes to be toleratedand this may so reduce constraints on the design of chromatographicsystems that several small unswept volumes may be rendered unnecessary.

The invention comprises a gas chromatographic system in which arestricted vent from the chromatographic gas stream is provided from anunswept volume.

The invention also comprises a union piece for connecting units of a gaschromatographic system through which union piece gas is to flow, whichunion piece encompasses an unswept volume, the union piece comprising arestricted vent from the chromatographic gas stream from the unsweptvolume.

The invention also comprises a sample injector for a gas chromatographicsystem which comprises a restricted vent from the chromatographic gasstream from an unswept volume.

The sample injector may comprise a carrier gas conduit, an injectionconduit for leading from a septum holder to the carrier gas conduit in asubstantially straight line to permit the passage down it of a hypoder'mic needle, and a restricted vent from the injection conduit to theatmosphere surrounding the sample injector.

Preferably the carrier gas conduit is provided on its output side withmeans to connect it to a chromatographic column which comprises arestricted vent from an unswept volume encompassed by the said means tothe atmosphere surrounding the sample injector.

If the restricted vents are supplied outside the apparatus with carriergas at a higher pressure than that applying to them within theapparatus, (for example, if they communicate withthe atmospheresurrounding the apparatus, by providing the apparatus with a vesselsurrounding the restricted vents, to which an elevated pressure isapplied), a flow of gas to the chromatographic gas stream will preventsample entering the unswept volume or carry it quickly back to the saidstream.

Preferably also the sample injector comprises means to heat the carriergas conduit. Suitably the carrier gas conduit and the injection conduitmay be formed in a metal block, preferably of high thermal conductivity,for example a brass block, which is provided with means for heating it,for example an electrical heating coil. I

The septum holder may be mounted on a sleeve for fitting in theinjection conduit, the sleeve being formed, if it projects past arestricted vent in the injection conduit, with a hole in its wallcorresponding to the inlet of the restricted vent.

The means to connect the sample injector to the chromatographic columnmay comprise an enlarged outlet portion of the carrier gas conduit withwalls adapted to form the socket of a cone and socket joint with a tubeleading to a chromatographic column, the inlet end of which is formed asa corresponding cone. A restricted vent is preferably provided to leadfrom between the tube and the enlarged part of the carrier gas conduiton the output side of the cone and socket joint to the atmosphere.

One form of sample injector according to the invention will now bedescribed with reference to the drawing which shows a cross-sectionthrough the sample in- .duit, into'which is inserted a sleeve 6 on whichis mounted a septum holder 7 containing a septum 8.

Restricted vents 11 and 12 lead respectively from sleeve 6 which has aport 13 passing through its wall in register with vent 1 1 and a gapbetween tube 9 and outlet 3. Both restricted vents are capillaries heldin position by conventional compression couplings l9 and 14 which act ondeformable metal olives 15 and 16.

A blind ended bore 17 is provided for accommodating an electricalcartridge heater (not shown) The inlet to conduit 2 is united bysoldering to the inserted end of an inlet tube 18 which is wound roundthe block 1 to form a helix.

The apparatus acts as follows:

The block 1 is heated by a heater in bore 17 and carrier gas passedthrough the helical tube 18, during which movement the carrier gasbecomes warm, and through conduit 2.

When it is desired to inject a sample, the needle of a hypodermicsyringe is pushed through the septum 8 and down sleeve 6 until its tipenters conduit 2. Liquid sample is then injected from the needle intoconduit 2 and the needle is withdrawn.

On contact with the heated carrier gas and the block out to theatmosphere through restricted vent 11 thus minimising tailing of thesample through diffusion back into conduit 2. Similarly, sample betweentube 9 and outlet 3 passes to the atmosphere through vent 12 thusminimising tailing.

it will be understood that in this specification a re stricted vent froma gas stream may in general function as a vent to the gas stream ifdesired if an appropriate pressure drop across it is provided.

I claim: I

1. A sample injector for a gas chromatographic systern which comprises ablock of high thermal conductivity comprising a carrier gas conduit anda sample injection conduit leading to the carrier gas conduit in asubstantially straight line, means to heat the block, a restricted ventfrom the sample injection conduit to an atmosphere surrounding thesample injector, a sleeve fitting into the injection conduit andpermitting free flow to the restricted vent in the injection conduit anda septum holder mounted on the sleeve and spaced away from the heatconducting block.

2. A sample injector as claimed in claim 1 in which the carrier gasconduit is provided on its output side with means to connect it to achromatographic column which means comprises a restricted vent from anunswept volume encompassed by the said means to an atmospheresurrounding the sample injector.

3. A sample injector as claimed in claim 1 in which the restricted ventfrom the injection conduit has 1 the sample vaporises and expands thusforcing part of the sample back into sleeve 6 and also back up conduit 2and between the tube 9 and outlet 3.

The sample in conduit 2 passes through in a block, but sample passinginto sleeve 6 passes, because of the positive pressure in the apparatus,up the sleeve and means to supply it with carrier gas at a higherpressure than that applying to it on its chromatographic gas streamside.

4. A sample injector as claimed in claim 3 which comprises a vessel towhich carrier gas at an elevated pressure may be applied and whichsurrounds the restricted vent.

5. A sample injector as claimed in claim 1 in which the block ofcomprises a metal of high thermal conductivity which is provided with anelectrical heating coil. s

1. A sample injector for a gas chromatographic system which comprises ablock of high thermal conductivity comprising a carrier gas conduit anda sample injection conduit leading to the carrier gas conduit in asubstantially straight line, means to heat the block, a restricted ventfrom the sample injection conduit to an atmosphere surrounding thesample injector, a sleeve fitting into the injection conduit andpermitting free flow to the restricted vent in the injection conduit anda septum holder mounted on the sleeve and spaced away from the heatconducting block.
 2. A sample injector as claimed in claim 1 in whichthe carrier gas conduit is provided on its output side with means toconnect it to a chromatographic column which means comprises arestricted vent from an unswept volume encompassed by the said means toan atmosphere surrounding the sample injector.
 3. A sample injector asclaimed in claim 1 in which the restricted vent from the injectionconduit has means to supply it with carrier gas at a higher pressurethan that applying to it on its chromatographic gas stream side.
 4. Asample injector as claimed in claim 3 which comprises a vessel to whichcarrier gas at an elevated pressure may be applied and which surroundsthe restricted vent.
 5. A sample injector as claimed in claim 1 in whichthe block of comprises a metal of high thermal conductivity which isprovided with an electrical heating coil.